The Structure of the Earth

Our present knowledge and some hypotheses concerning the core of the earth, the mantle and the crust are summarized. The earth consists of a core with a radius of about 3450 km, a mantle, and surface layers which vary from place to place. According to the usual theory, the core consists mainly of fluid iron, but recent theories point to a possibility that it contains a relatively large percentage of hydrogen. Another change in properties is indicated at a depth of 900 to 1000 km. Results concerning the earth's uppermost layer are discussed. These include irregularities at a depth of about 80 km, where the velocity of longitudinal as well as transverse waves decrease slightly but sufficiently to cause a "shadow zone" for both types of waves at the surface of the earth; this decrease in wave velocity is due to the high temperature which approaches or even surpasses the melting point of the material at depths of about 80 km and below. Other phenomena which are mentioned include "roots of mountains" and the "granitic layer." Data. concerning the density, the elastic constants, and plasticity are given.